This invention relates to an optical autocorrelator and more especially to such a device based on two-photon absorption.
Optical autocorrelators are used for monitoring the width and shape of very short duration optical pulses, i.e. pulse-widths down to a few femtoseconds. Optical autocorrelators are potentially very useful in high data rate optical communication systems (those operating at ≧40 Gbit/sec) where they could be used for monitoring pulse quality and timing jitter in data bit interleaved systems.
Referring to FIG. 1 there is shown a schematic representation of a known optical autocorrelator arrangement which comprises an optical splitter 2, a reference path 4, a variable path delay 6, a non-linear mixing medium 8 and an optical detector 10. An input optical pulse 12 is split by the optical splitter 2 into two signals, sometimes termed channels, 14a, 16a which respectively pass along the reference path 4 and variable path delay 6. Typically the reference path 4 comprises a fixed retro-reflector 18 whilst the variable path delay 6 comprises a moveable retro-reflector 20 which is mounted on a mechanically oscillating stage, such as a loudspeaker. Movement of the retro-reflector 20 is indicated by a double headed arrow 21. The two optical signals 14a 16a are reflected by their respective reflectors 18, 20 and the reflected signals 14b, 16b are mixed together in the non-linear mixing medium 8 to produce a non-linear mixing product which is detected by the optical detector 10. The detected signal is plotted as a function of relative path delay between the two channels and represents an autocorrelation trace of the input optical pulse 12 which can be related to the input pulse-width and shape.
Typically the non-linear mixing element 8 exploits second-harmonic generation (SHG), but other non-linear mixing elements have been proposed which use processes such as two-photon fluorescence (TPF) and two-photon absorption (TPA).
TPA is a non-resonant, non-linear optical process that is observed for photons with energy less than the semiconductor band-gap Eg, but greater than Eg/2. The process occurs when an electron is excited from the valence to the conduction band via an intermediate virtual state and thus requires two photons. This intermediate state can be any state in any band, although the transition probability is highest when the energy difference between the states involved is smallest; that is, when the intermediate state lies closest to the upper valence band or lower conduction band. One example of a combined mixing and detector element is an AlGaAs/GaAs optical waveguide having a pair of electrodes on the waveguide to measure the photocurrent generated in the waveguide by the two optical signals.
A disadvantage of this type of optical autocorrelator is the difficulty of integrating the moveable reflector, optical splitter and non-linear mixing element.
Optical autocorrelators have also been proposed which rely upon surface-emitting second-harmonic generators (SESHG). An SESHG autocorrelator comprises a waveguide into which identical optical signals are input into opposite ends of the waveguide such as to generate counter-propagating signals. As a result of the counter-propagating waves these generate, via the second-order optical non-linearity of the waveguide material, a second-harmonic (SH) signal that is emitted from the surface of the waveguide in a direction normal to its axis. The spatial distribution of the surface-emitted SH signal along the waveguide constitutes an autocorrelation trace and this is typically imaged onto a charged-coupled device (CCD) camera using bulk optics. It has been further proposed to monolithically integrate an array of photodiodes along the length of the waveguide. To eliminate the need to generate the identical input signals it has been proposed to input a single optical signal into one end of the waveguide and rely on reflection from the opposite end facet to generate the counter-propagating optical signal.
It has also been demonstrated to couple identical signals into opposite ends of the p-i-n waveguide such that they counter-propagate. Such an arrangement eliminates the need for a variable path delay thereby making it possible to fabricate the autocorrelator as an integrated device.